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New nickel(ii) complexes of the general formula LNi(Cp)X (L is an N-heterocyclic carbene (NHC) ligand of the 1,2,4-triazole or imidazole series; Cp is the cyclopentadienyl anion; X = Cl, I) are reported. In these complexes, the NHC ligands (L) contain an alkylamino group at the 3 or 4 position of the heterocycle. The synthesized complexes and structurally similar complexes without an alkylamino group were tested for catalytic activity in the Suzuki—Miyaura reaction. The introduction of an alkylamino group into the NHC ligand leads to the enhancement of the catalytic activity of complexes with N,N′-diaryl-substituted NHC ligands of the imidazole series and a decrease in the activity of the complexes with N,N′-dialkyl-substituted NHC ligands of the 1,2,4-triazole series.

C–H functionalization in the area of fine organic synthesis is dominated by noble metal catalysts, which represent the most expensive and least sustainable options. Sustainable C–H functionalization may involve Ni catalysts. However, Ni(0) complexes are unstable under regular conditions and are more difficult to obtain as compared to Pd(0) or Rh(I). In the present study, a facile method for Ni0/NHC-catalyzed C–H alkylation and alkenylation of heteroarenes with alkenes and internal alkynes is presented. This method relies on the in situ generation of Ni0/NHC complexes from air-tolerant bench-stable precursors, Ni(Cp)2, NHC·HCl salts and sodium formate. The optimized catalytic system demonstrates broad substrate scope and high selectivity (>60 products were obtained in up to 99% isolated yield). The approach represents a user-friendly alternative for air-sensitive and labile (NHC)Ni0 and Ni(COD)2 precatalysts or complexes. The intermediates involved in the catalytic system were investigated and possible decomposition routes were mapped with NMR and ESI-MS. Rational control over the catalyst decomposition pathways further strengthens the sustainability of the procedure.

Recently, the dynamic nature of the metal-NHC bond has been proposed and the key role of chemical evolution in changing the nature of catalytically active sites is now an emerging topic. A comparative analysis of the ketone α-arylation reaction with aryl halides, catalyzed by M/NHC complexes, was carried out in the present study and showed a fundamental difference in the behavior of the catalytic system for M = Ni and Pd. In situ evolution of Ni/NHC complexes with cleavage of the Ni-NHC bond leads to complete deactivation of catalytic systems, regardless of the nature of the aryl halide ArX (X = Cl, Br, I). However, upon Pd/NHC catalysis, the cleavage of the Pd-NHC bond causes deactivation only in the case of aryl chlorides. In the reactions of more active aryl iodides and aryl bromides, NHC-disconnected Pd species, formed as a result of the chemical transformation of Pd/NHC complexes, can provide effective catalysis in the arylation reaction under study. New catalytic systems based on Pd/NHC and Ni/NHC complexes generated in situ from stable imidazolium salts, IPr·HCl and IPr*OMe·HCl, and Pd(OAc)2 (0.1 mol%) or NiCl2Py2 (5 mol%) were developed for the selective α-arylation of methylaryl ketones (Pd-catalysis) and other ketones less prone to aldol-crotonic condensation (Ni-catalysis). The present study has shown that the different effects of the metal-NHC bond cleavage should be taken into account for the efficient choice and optimization of catalytic systems to carry out arylation reaction with various aryl halides.

A DFT study was carried out of how the nature of metal, the oxidation state of the metal (0 and +2), as well as the structures of N-heterocyclic carbene (NHC) and other ligands influence the heterolytic dissociation energies of the metal-ligand bond in the complexes M-NHC (M = Ni, Pd, Pt). It was shown that a change in the oxidation state of the metal can be followed by a considerable change in the M-NHC bond dissociation energy (up to nearly 21 kcal mol−1), which is also strongly influenced by the ligand in the trans-position to NHC.

Catalytic activity of nickel(ii) and palladium(ii) N-heterocyclic carbene (NHC) complexes derived from imidazole, benzimidazole, and 1,2,4-triazole was comparatively evaluated in the cross-coupling reactions of aryl halides with arylboronic acids. Readily available nickel bis-NHC complexes (NHC)2NiX2 (X = Cl, Br, or I) exhibited the activity comparable to that of the structurally related palladium complexes and, consequently, can be applied as efficient catalysts for the Suzuki—Miyaura reaction.